Japanese Patent Laying-Open No. 10-066383 discloses a drive control device for a permanent magnet synchronous motor (PM motor). The drive control device includes a boost circuit boosting a battery voltage prior to be supplied to an electric power converter and in response to a command, and means for causing the boost circuit to perform a boost operation such that a boosted voltage value is higher than a voltage on a DC (direct current) terminal side of the electric power converter that is required to achieve a target operation point of the PM motor.
In the drive control device, when the target operation point of the PM motor is located at a higher rotation side than an outputtable range with the present battery voltage, the battery voltage is boosted such that the target operation point is included in the outputtable range. As a result, extension of the outputtable range when the PM motor performs power running, which has been achieved by field weakening control, can be achieved without causing a loss due to a field weakening current and a reduction in system efficiency due to the loss.
In the technique disclosed in Japanese Patent Laying-Open No. 10-066383 described above, the boost circuit has a configuration including two transistors connected in series in a forward direction between DC terminals of an IPM (Intelligent Power Module), two diodes connected in antiparallel with these transistors, respectively, and a boost reactor having one end connected to a connection point between the two transistors and the other end connected to a battery side. In such a configuration, a switching operation of the two transistors is controlled in response to a command from a controller on a boost ratio for the boost circuit.
In a configuration using a boost circuit as disclosed in Japanese Patent Laying-Open No. 10-066383, when a transistor of the boost circuit performs a switching operation, an AC (alternating current) current that depends on a switching frequency thereof (hereinafter also referred to as a ripple current) occurs. Due to the ripple current, a vibration occurs in a battery, which may cause noise.
Accordingly, the present invention has been made to solve the above-mentioned problem, and one object of the present invention is to provide a load driving device capable of suppressing vibration noise in a power supply caused by a ripple current.